Fuel cells are a clean, efficient and an environmentally responsible power source for vehicles and various other applications. The fuel cell is under intense development as a potential alternative for the traditional internal-combustion engine used in modern vehicles. In proton exchange membrane (PEM) type fuel cells, a thin solid electrolytic membrane having an electrode with catalyst adjacent both sides forms a membrane electrolyte assembly (MEA). The MEA generally also includes porous conductive materials known as gas diffusion media (DM), which abut and distribute reactant gases to the anode and cathode. Hydrogen is supplied as fuel to the anode where it reacts electrochemically in the presence of catalyst to produce electrons and protons. The electrons are conducted by circuit from the anode to the cathode, and the protons migrate through the electrolyte to the cathode where oxygen reacts electrochemically in the presence of catalyst to produce oxygen anions. The oxygen anions react with the protons to form water as the fuel cell reaction product.
PEM fuel cells are typically connected in series, stacked one on top of the other to form a fuel cell stack. A fuel cell stack is ordinarily assembled under compression in order to seal the fuel cells and to secure and maintain a low interfacial electrical contact resistance between the reactant plates, the gas diffusion media, and the catalyst electrodes. The interfacial contact resistances in a PEM fuel cell stack decrease substantially with increasing compression loading. A desired compression load on the fuel cell stack typically ranges from about 50 to about 400 psi, and is maintained by a compression retention enclosure housing the fuel cell stack.
To establish the desired retained compressive force, the fuel cell stack is placed in a press, an over-compression is applied, a compression retention system is engaged, the press is released, and the stack is held under a pressure retained by the engaged compression retention system. In some cases the compression retention system is thereafter placed into a separate enclosure for environmental sealing, and in other systems, enclosing side panels may provide any necessary sealing. The enclosed fuel cell stack is then positioned in the vehicle.
Although various compression retention systems are known in the art, the function of the system is generally limited to compression retention and sealing. Systems which additionally perform a structural function by integration with vehicle mounting are not known.
In some existing compression retention system designs, tie rods interconnecting rigid end plates may be used to apply and maintain a compressive force on the fuel cell assembly. The plurality of fuel cells or fuel cell stack to be compressed is interposed between a pair of rigid end plates, and the end plates are then compressed together by tie rods that extend through or around the end plates and impart a compressive force on the end plates. Compressive force is retained by securing the position of the tie rods. Tie rods typically extend beyond the surface of the end plates and thereby increase the volume of the stack structure.
Additional compression retention solutions include, for example, fuel cell side plates having spring elements incorporated therein to control tensile compliance (U.S. Patent Appl. Pub. No. 2006/0040166), and a mechanism utilizing a number of compression bands circumscribing the end plate assemblies in order to secure the stack in a compressed state (U.S. Pat. No. 5,789,091).
Further in accordance with conventional fuel cell stack compression enclosures, the dry end is fixedly located by bolting in position to the endcaps. Since assembled stack height varies, in some cases by as much as 5-10%, the wet end unit floats to accommodate the variation. The wet end is the platform in which the lower end unit, reactant manifolds and balance of parts build from, changes, and this drives design complexities in the balance of the plant. Slip joints may be required to tolerate stack height variation.
Component parameters of the compression retention enclosures are typically set such that accommodation for variation between fuel cell stack build height based on fuel cell stack component variance or vehicle balance of plant component parameter variance is problematic.
It would be advantageous to provide a compression retention system which accommodates fuel cell stack height variation at the dry end and which is structurally integrated with vehicle mounting.